1. Field of the Invention
The present invention relates to polymeric diacetylene thin films which are suitable for nonlinear optical applications and their method for preparation. In another aspect the invention relates to photo-deposition of thin polydiacetylene films from solutions of diacetylene monomer that exhibit outstanding third-order optical nonlinearity.
2. Description of Related Art
The development of advanced optical materials that can focus, modulate, multiplex, transmit, receive, demultiplex and demodulate optical signals, will be the key to the realization of economical and reliable optoelectronic systems for optical signal processing and computing applications. Efforts have focused on the development of hybrid and monolithically integrated devices and systems in for example III-V material systems. A number of technological related issues, however, currently impede further progress since III-V semiconductors are incapable of producing the large index modulations that are required to create multiplex phase gratings, which constitute one of several important building block in very large scale optically interconnected systems. In addition, dielectric constants of these materials are dispersive and relatively high when compared with polymeric materials. Further, device yields have been relatively low while cost associated with the growth and processing of related microstructures have been high. Hence the development of new, low cost materials that can be processed into microstructural optical components, will be invaluable to the future optoelectronic integration effort.
The solid state polymerization of diacetylenes has found interest by many in the literature since the nonlinear optical properties of some polydiacetylenes are attractive and third-order NLO properties have been reported. It is also reported that polydiacetylenes are materials with difficulty in processing since the majority are insoluble and infusible, and the materials do not form large single crystals. Presently used methodologies for processing of polydiacetylenes are not simple and are not necessarily suitable for applications and devices. Vapor-phase deposition of some sublimable diacetylenes on quartz plate to obtain crystalline films has been reported. However, polymerizability of diacetylenes depends on their crystalline structures. Polymers of diacetylenes in the molten state can yield amorphous polymer forms whereby the orientation of the polydiacetylenes chains are random.
The polydiacetylenes are of the most interesting materials for optical applications due to their highly conjugated electronic structures. However, they are also difficult materials to be processed into thin films. Various membrane techniques are too complicated for obtaining films for practical uses, and other methods such as vapor deposition are limited to very few particular diacetylenes. The search continues for classes of polydiacetylenes in order to obtain thin films of polydiacetylenes without complex methods of film preparation. Various attempts to disperse homogeneously polydiacetylene crystals in a transparent polymer film does not relieve complexities of forming suitable thin films of polydiacetylenes.
Typically, growth of thin films of polydiacetylenes involve growth of thin crystalline films of diacetylenes monomer followed by topochemical polymerization of these films in the solid-state. This process mostly results in small domains of crystalline polymeric films unacceptable for device applications. The solid-state polymerization production of thin crystalline films of diacetylene is generally achieved by radiation with UV light. One drawback of this technique is that growth of large area monocrystalline thin films possessing high optical quality suitable for devices is not often possible. Thus polydiacetylene thin films are frequently polycrystalline in nature, which renders them translucent or even opaque. Such films greatly undermine their utility for practical applications because of light scattering due to crystal grain boundaries and other defects. In order to be suitable for devices, films with high optical quality, good transparency, minimum scattering centers and the like are required. Clearly an improvement in the existing technology is needed to make monomeric and polymeric diacetylene thin films that can be practical for device applications.
Recent publications surrounding this subject involve use of photopolymerization of a poled polymer to stabilize an orientated polymer against relaxation and loss of good second-order nonlinearity. Other methodologies for production of wave-guiding arrays have been shown to include spin-coting techniques which are also classical methods but involve numerous steps. The spin coating technique produces amorphous thin films and is generally confined to low molecular weights, hence lower optical quality polymers. There are a myriad of papers which discuss the classical methods for providing diacetylene technology for use and devices requiring third-order optical nonlinearity. However, these classical methods lack either efficiency, simplicity and process control which singly or jointly can result in an unsatisfactory product for device usage.